1. Field of the Invention
The present invention relates to optical signal noise ratio (OSNR) measurement in an optical communication system.
2. Description of the Related Art
Corresponding to an increased volume of information communicated in recent years, research and development related to high capacity, low cost fiber optic communications systems has surged. To further increase capacity and to lower cost, wavelength division multiplexing (WDM), a method for sending multiple wavelengths (that are multiplexed signals) along a single optical fiber, has been and continues to be extensively researched and developed.
Channel spacing, an index of multiplexing, has been standardized by the Telecommunications Standardized Sector of the International Telecommunications Union (ITU). Currently among standard WDM systems, a typical system multiplexes a 10-gigabit per second (Gbps) signal which is the signal transmission capacity for one channel having a 100-gigahertz (GHz) (approximately 0.8 nano meters (nm)) spacing or a 50-GHz (approximately 0.4 nm) spacing.
In WDM systems, an erbium doped fiber amplifier (EDFA) is commonly used as a repeater to offset optical fiber line loss. In systems employing EDFA, amplified spontaneous emission (ASE) is generated becoming noise causing bit error rate (BER) increases. As such, the optical signal noise ratio (OSNR) evaluation becomes important.
Since multiple channels transmit simultaneously, OSNR for the receiving end (after transmission) of each channel differs for each channel. Additionally, the BER of each channel also varies. As such, the quality of transmission among channels becomes unequal. Hence, to optimize the transmission level of each channel such that the transmission quality becomes equivalent, preemphasis is commonly employed. Japanese Patent Application Laid-Open Publication No. H6-69891 is an example of such an application.
In the process of preemphasis, since the level of each channel on the transmission side is determined based on OSNR, OSNR must be accurately measured. Spectrum monitors are employed to measure the spectral and ASE component of the signal (below, a “spectrum analyzer” is also given as an example of a spectrum monitor) and calculation based on these measurements provides the highest accuracy.
Accompanying increased WDM optical communication transmission capacity, per channel bit rates have increased (improved speed), and channel spacing has decreased. Along with these advancements, the following problems related to measurement by spectrum analyzer have arisen.
In order for the spectrum analyzer to accurately measure OSNR, the extent to which the spectrum of signal component and component of noise (in this case, primarily ASE) can be accurately separated and measured becomes very important. The entire signal spectrum must be within the resolution band of the spectrum analyzer. As such, the total power of the signal component can be accurately measured. Further, with regard to the ASE component, extraneous components (e.g., signal spectrum) must be blocked from entering the resolution band of the spectrum analyzer.
FIG. 12 is a graph illustrating a WDM signal spectrum for a 50-GHz channel spacing in which the signal for each channel is 10 Gbps and modulated as non-return-to-zero (NRZ). The x-axis represents optical frequency f, while the y-axis represents a measured spectral power. Reference characters 1201, 1202, and 1203 indicate signal spectrums of each channel. Reference character 1204 indicates the ASE component that arises from the EDFA. Furthermore, the signal spectrum 1201 is the signal spectrum of the target channel for which the OSNR measurement is sought.
As illustrated in FIG. 12, the bit rate for one channel is high (each signal spectrum width increases). Further, if channel spacing decreases, the ASE component 1204 is buried by the bases of the signal spectrums 1201, 1202, and 1203. Therefore, the ASE component 1204 cannot to be accurately measured.
At the same time, if the resolution is increased too much in order to enhance accuracy of ASE component measurement, the entire signal spectrum of the target channel (the signal spectrum 1201) does not fall within the resolution band, leading to the problem of not being able to accurately measure the signal spectrum.
FIG. 13 is a graph illustrating a WDM signal spectrum for a 50-GHz channel spacing. The signals 1201, 1202, and 1203 for each channel are depicted as optical continuous waves (CW). As illustrated in FIG. 13, the bit rate becomes zero when each of the signals is an optical CW and hence the width of the signals 1201, 1202, and 1203 diminishes and the problem of the not being able to measure the ASE component because of the signals 1201, 1202, and 1203 burying the ASE component does not arise.
FIG. 14A is a graph of a 40-Gbps “return to zero-differential quadrature phase shift keying” (RZ-DQPSK) signal spectrum measurement calculation example (resolution 0.1 nm). FIG. 14B illustrates the signal measurement calculation in FIG. 14A over a 100-GHz channel spacing for three channels (resolution 0.1 nm). In both FIG. 14A and FIG. 14B, as an example of modulation, the signal spectrum results calculated by the RZ-DQPSK method are illustrated (resolution 0.1 nm). RZ-DQPSK is a four-phase modulation in which differential coding of a data signal is achieved and has a signal format that returns to zero within the bit interval.
In both FIG. 14A and FIG. 14B, the x-axis represents the relative frequency of the RZ-DQPSK signal and the y-axis represents the measured relative optical power. In the figures, reference character 1401 depicts the signal spectrum of the target channel (corresponds to the signal 1201). Furthermore, in FIG. 14B, reference characters 1402 and 1403 depict signals of the channels adjacent to the target channel (corresponds to the signals 1202 and 1203 above).
As apparent by FIG. 14B, if the bit rate for one channel is high (40 Gbps) and in addition if the channel spacing is narrow (100 GHz), the bases of the signals 1401, 1402, and 1403 of each channel overlap each other. The problematic ASE component is not illustrated. In this case, the resolution of the spectrum analyzer is 0.1 nm.
FIG. 15A, FIG. 15B, and FIG. 15C depict the signal spectrum illustrated in FIG. 14B and ASE component.
In each of the figures, FIG. 15A, FIG. 15B, and FIG. 15C, the power of the ASE component is different and as such, the measured value of OSNR also varies among figures. In FIG. 15A, FIG. 15B, and FIG. 15C, reference character 1501 represents the actual OSNR and reference character 1502 represents the measured OSNR. Reference character 1503 depicts the ASE component.
In the example shown in FIG. 15A, the actual OSNR 1501 is 40 decibels (dB) and the measured OSNR 1502 is 23.5 dB. In the example shown in FIG. 15B, the actual OSNR 1501 is 30 dB and the measured OSNR 1502 is 22.2 dB. In the example shown in FIG. 15C, the actual OSNR 1501 is 20 dB and the measured OSNR 1502 is 16.4 dB.
Furthermore, reference character 1504 depicts an optical CW for each channel. Compared to the optical CW signal 1504, the modulated signals 1401, 1402, and 1403 have a greater spectrum width and lower spectral power peak.
Hence, in addition to the bit rate for one channel being high, if the channel spacing is also narrow, the bases of each channel become overlapped and the signal spectrum of the ASE component 1503 becomes buried. Furthermore, the modulated signals 1401, 1402, and 1403 have lower peak powers compared to the optical CW 1504. As such, it becomes apparent that the actual OSNR 1501 can not be accurately measured.